Display devices have typically used cathode-ray tubes (CRT). Presently, much effort has been made to study and develop various types of flat panel displays, such as liquid crystal display (LCD) devices, plasma display panels (PDP), field emission displays, and electro-luminescence displays (ELD), as a substitute for CRT. Of these, flat panel displays and LCD devices have advantages, such as high resolution, light weight, thin profile, compact size, and low power supply requirements.
In general, an LCD device includes two substrates that are spaced apart and face each other with a liquid crystal material interposed between the two substrates. The two substrates include electrodes that face each other such that a voltage applied between the electrodes induces an electric field across the liquid crystal material. Alignment of the liquid crystal molecules in the liquid crystal material changes in accordance with the intensity of the induced electric field, thereby changing the light transmissivity of the LCD device. Thus, the LCD device displays images by varying the intensity of the induced electric field. LCD devices are non-luminous display devices in that they cannot display images without some light source (e.g., ambient light or a backlight).
A backlight unit for the LCD device is categorized as a direct type or an edge type. The direct type backlight unit has a plurality of lamps directly below a liquid crystal panel, and the edge type backlight unit has a lamp at a side of the liquid crystal panel. The direct type backlight unit uniformly supplies light to the liquid crystal panel and, in particular, the large sized liquid crystal panel.
As illustrated in FIG. 1, a liquid crystal display module includes a liquid crystal panel 13, a backlight unit 20, a main support 17, a bottom cover 27 and a top cover 11.
The backlight unit 20 includes a plurality of lamps 23 arranged in parallel below the liquid crystal panel 13, a reflector 21 and a plurality of optical sheets 19 including prism and diffusion sheets. The plurality of lamps 23 are retained by a pair of lamp guides 25 combined with the bottom cover 27. The main support 17 surrounds peripheral portions of the backlight unit 20 and is combined with the bottom cover 27. The liquid crystal panel 13 is laid on the backlight unit 20. The top cover 11 is combined with the bottom cover 27 such that the backlight unit 20 and the liquid crystal panel 13 are retained thereby. The lamp 23 includes a CCFL (cold cathode fluorescent lamp) having electrodes at both ends. The electrodes of the CCFL are supplied with driving voltages to supply light to the liquid crystal panel 13.
FIG. 2A is a plan view illustrating lamps and a driving circuit for lamps of a direct-type backlight unit, and FIG. 2B is a plan view illustrating lamps and another driving circuit for lamps of a direct-type backlight unit.
As illustrated in FIG. 2A, a plurality of lamps 23 are arranged in parallel, and the lamp 23 has a glass tube. The glass tube is filled with a discharge gas including an inert gas and mercury (Hg). A fluorescent material layer is formed on an inner surface of the glass tube. First and second (left and right) electrodes are formed on ends of the glass tube.
The lamp 23 of FIG. 2A is operated by a high-high method. In other words, each of the both electrodes is supplied with a high voltage. When the lamp 23 is operated by a high-low method i.e., high and low voltages are supplied to the electrodes, as a length of the lamp 23 increases, a power operating the lamp 23 increases. Accordingly, when the lamp 23 is operated with the high and low voltages, the lamp 23 is problematic for the large sized LCD device. Therefore, high voltages are supplied to the both electrodes of the lamp 23 to operate the large sized LCD device. To operate the lamp 23 with high voltages, a ground portion (ground terminal) is disposed at a center portion of the lamp 23. The high voltages supplied to the lamp 23 are AC (alternating current) voltages.
To supply the AC high voltage, a driving circuit is used. The driving circuit includes a plurality of inverters 33 and a plurality of transformers 31 each connected to each electrode of the plurality of lamps 23. The inverter 33 converts a source voltage into an AC voltage, and the transformer 31 increases the AC voltage from the inverter 33 so as to output a AC high voltage. The AC high voltage outputted from each transformer 31 is supplied to each electrode of the lamps 23, and thus the lamps 23 are operated independently from each other. As the lamps 23 are operated separately, transformers 31 and inverters 33 corresponding to the both electrodes of the lamps 23 are required. Accordingly, as a size of the LCD device increases, and a number of lamps 23 increases, and also, a number of the transformer 31 and the inverters 33 increase. Therefore, fabrication cost increases.
As illustrated in FIG. 2B, a plurality of lamps 23 are arranged in parallel and operated by a high-high method, as similarly in FIG. 2A. Each lamp 23 has first and second (left and right) electrodes. Each of the first and second electrodes is supplied with high voltages. The first electrodes are supplied with the high voltage from one or two transformers and inverters 31 and 33, and the second electrodes are supplied with the high voltage from one or two transformers and inverters 31 and 33. In other words, the driving circuit of FIG. 2B includes the fewer transformers 31 and inverters 33 than the driving circuit of FIG. 2A, and thus fabrication cost is reduced.
However, lamp currents flowing in the outermost lamps 23 (that is, the lamps at the top and the bottom of a plurality of lamps arranged in parallel) are distorted due to structure and thermal distribution of the LCD device. With regard to the structure of the LCD device, since upper and lower portions of the bottom cover (27 of FIG. 1) are made of metal are bent toward a front, the bent upper portion forms a parasitic capacitor with the uppermost lamps 23 adjacent to the bent upper portion, and the bent lower portion forms a parasitic capacitor with the lowermost lamps 23 adjacent to the bent lower portion. Since the parasitic capacitor causes leakage of the lamp currents, the lamp currents in the outermost lamps are distorted.
In addition, with regard to the thermal distribution of the LCD device, there is difference of temperature between the upper portion and the lower portion of a space where the lamps are arranged. When a user looks at the LCD device displaying an image, the lowermost lamps face a ground and the uppermost lamps face a sky. When the lamps 23 are operated, heat is generated by the transformers 31 at the center portion of the LCD device. The generated heat moves toward an upper portion of the space where the lamps are arranged. Therefore, the upper portion of the space has a higher temperature and the lower portion of the space has a lower temperature. Non-uniformity of the thermal distribution in the space causes distortion of the lamp currents in the lamps.
As a result, due to the structure and thermal distribution of the LCD device, the lamp currents in the outermost lamps and, in particular, in the lowermost lamps, are distorted.